1. Field of the Invention
The present invention relates to an optical disk apparatus for recording, reproducing or erasing information on an optical disk that acts as an optical information medium.
2. Description of the Related Art
The optical storage technology that employs a optical disk with pit patterns as a high-density, large-capacity recording medium has been put into practical use while expanding its applications to digital versatile disks (DVD), video disks, document file disks, and data files. The functions required for recording/reproducing information successfully and with high reliability on an optical disk by a finely focused light beam (e.g., with a diameter of 1 μm or less) are classified into three major categories: a focusing function for forming a diffraction-limited tiny spot, focusing control (focus servo) and tracking control functions of an optical system, and a pit signal (information signal) detecting function.
To improve the recording density of an optical disk further, an increase in the numerical aperture (NA) of an objective lens has been studied recently. The objective lens focuses a light beam on the optical disk to form a diffraction-limited tiny spot. However, spherical aberration, which is caused by an error in thickness of a base material for protecting a recording layer of the optical disk, is proportional to the fourth power of NA. Therefore, when NA is increased, e.g., to 0.8 or 0.85, the spherical aberration becomes significantly large. Thus, a means for correcting the spherical aberration is essential to the optical system. FIG. 13 shows an example of such an optical system.
Referring to an optical pickup 11 in FIG. 13, numeral 1 denotes a radiation source such as a laser source. Alight beam 20 (a laser beam) emitted from the laser source 1 is converted into parallel light by a collimator lens 3, passes through a liquid crystal aberration correcting element 4, and enters an objective lens 5 to be focused onto an information recording plane of an optical disk 6. The light beam reflected from the optical disk 6 retraces the same optical path so as to be condensed by the collimator lens 3 and is directed into photodetectors 9, 10 by a light separating device such as a diffraction element 2. Servo signals (i.e., a focus error signal and a tracking error signal) and information signals are generated from output signals of the photodetectors 9, 10. Here, the NA of the objective lens 5 is as large as 0.8 or more. An actuator 7 includes a driving means, such as coils and magnets, and performs focusing control for positioning the objective lens 5 in the direction parallel to an optical axis and tracking control for positioning it in the direction perpendicular to the optical axis.
A transparent base material (not shown) is formed on the information recording plane of the optical disk 6 on the side of the objective lens 5 and serves to protect information. Since differences in thickness and refractive index of the transparent base material cause spherical aberration, the liquid crystal aberration correcting element 4 corrects a wavefront of the light beam to provide optimum reproduction signals. The liquid crystal aberration correcting element 4 has a transparent electrode pattern made of indium-tin-oxide (ITO) alloy or the like. The in-plane refractive index distribution of the liquid crystal aberration correcting element 4 is controlled by applying a voltage to the transparent electrode so as to modulate the wavefront of the light beam.
FIG. 14 shows an optical disk apparatus 116. Referring to FIG. 14, numeral 8 denotes an aberration correcting element driving circuit 8 that applies a voltage to the liquid crystal aberration correcting element 4, and 118 denotes a control circuit that receives a signal from the optical pickup 11 and controls and drives the actuator 7, the aberration correcting element driving circuit 8, and the laser source 1. The control circuit 118 causes the laser source 1 to emit a light beam and controls the position of the objective lens 5 based on the signal from the optical pickup 11. Moreover, it drives the aberration correcting element driving circuit 8 to improve information signals from the optical pickup 11.
In addition to the above example, JP 2000-131603 A also discloses an optical system for the optical pickup 11, which is illustrated in FIG. 15.
FIG. 15 shows the components of the optical system other than a laser source, a collimator lens, and a photodetector. A light beam that has been converted into parallel light by a collimator lens passes through an aberration correcting lens group 201 and is focused on an optical disk 6 by an objective lens group 202. The aberration correcting lens group 201 includes a negative lens group 21 and a positive lens group 22. The objective lens group includes an objective lens 302 and a forward lens 301. The space between the negative and positive lens groups 21, 22 is changed to correct spherical aberration in the entire optical system. To change the space between the two lens groups, e.g., a driving portion 25 that shifts the negative lens group 21 in the optical axis direction can be used. The driving portion 25 may be formed, e.g., of a voice coil, a piezoelectric element, an ultrasonic motor, a screw feeder, or the like.
In the above configuration, spherical aberration is corrected so as to improve the quality of information signals on the assumption that the optical disk 6 has a single information recording plane and focusing control is performed stably on the information recording plane. For the DVD standard that uses an objective lens having an NA of 0.6, a two-layer disk with two information recording planes is employed. Therefore, the two-layer disk structure as well as a larger NA is effective in increasing recording capacity per optical disk.
As shown in FIG. 16, two-layer disk 6 includes a base material 62, an L0 layer (a first recording layer) 63, an intermediate layer 65, an L1 layer (a second recording layer) 64, and a protective layer 66 to form the back of the optical disk, which are stacked in this order from the optical pickup side. The base material 62 and the intermediate layer 65 are transparent media of resin or the like. Since the intermediate layer 65 is between the L0 layer 63 and the L1 layer 64, the thickness measured from the surface 61 of the optical disk 6 on the optical pickup side to the second recording layer (L1 layer) 64 is larger than that to the first recording layer (L0 layer) 63 by the thickness of the intermediate layer 65. Such a difference in thickness causes spherical aberration. However, the magnitude of the spherical aberration can be tolerated by the optical system of the DVD standard that includes an objective lens having an NA of 0.6. Therefore, it is possible to record/reproduce information without correcting the spherical aberration.
When NA is increased to 0.8 or more so as to achieve a further improvement in the recording density of an optical disk, spherical aberration caused by the thickness of the intermediate layer 65 cannot be ignored. In other words, the correction of spherical aberration is indispensable for recording/reproducing information on both of the recording layers. As described above, increasing NA to 0.8 or more requires a means for correcting spherical aberration even if information is recorded/reproduced on a single recording layer. Thus, as a matter of course, it is necessary to correct spherical aberration optimally at each of the recording layers when information is recorded/reproduced on a two-layer disk as shown in FIG. 16. This can eliminate the spherical aberration caused by the thickness of the intermediate layer.
JP 10(1998)-188301 A discloses the correction of spherical aberration that is performed before operating focusing control on an information recording plane. FIG. 17 shows this configuration. An objective lens 302 is held by a holder 305, and a forward lens 301 is held on the holder 305 via a second drive means 304. Therefore, a first drive means 303 that supports the holder 305 drives both of the forward lens 301 and the objective lens 302 in a focusing direction. The second drive means 304 drives the forward lens 301 relative to the objective lens 302 in the focusing direction. The space between the forward lens 301 and the objective lens 302 can be changed by driving the forward lens 301 in the focusing direction with the second drive means 304, thus correcting spherical aberration.
In this configuration, the first drive means 303 drives the forward lens 301 and the objective lens 302 together in the focusing direction. Therefore, these lenses are prone to deviate from the center and tilt, which makes it difficult to satisfy the strict tolerance of positioning accuracy for the lenses 301, 302.
Next, the problems of the aberration correcting lens group including two lens groups, i.e., the positive lens group and the negative lens group, will be described. FIGS. 18A and 18B are schematic views showing the aberration correcting lens groups located with their optical axes extending in a horizontal direction and in a vertical direction, respectively.
The aberration correcting lens group 201 located with its optical axis 201a horizontal is explained by referring to FIG. 18A. As shown in FIG. 18A, the positive lens group 22 is fixed and held by a stationary portion 26, while the negative lens group 21 is held by a lens holder 24. The lens holder 24 is held by the stationary portion 26 via a plurality of elastic wires 27. Therefore, the negative lens group 21 is held by the stationary portion 26 in a cantilever supporting fashion. A driving portion (not shown) that shifts the negative lens group 21 held by the lens holder 24 in the direction of the optical axis 201a is provided to change the space between the positive and negative lens groups 22, 21, thus correcting spherical aberration.
When the aberration correcting lens group 201 is located with its optical axis 201a horizontal, there is no problem because the negative lens group 21 is at the position Y0 in the direction of the optical axis 201a as designed, and the space between the positive and negative lens groups 22, 21 also is kept at a designed value A.
Depending on the orientation of an optical disk apparatus or the design of an optical pickup, the aberration correcting lens group 201 may be located with its optical axis 201a vertical. This configuration is explained by referring to FIG. 18B. As shown in FIG. 18B, the position of the negative lens group 21 in the direction of the optical axis 201a is shifted to Y1 due to the gravitational displacement of the negative lens group 21 and the lens holder 24. The position Y1 deviates from the position Y0, which is not affected by the gravitation displacement of the negative lens group 21 and the lens holder 24, by a distance α in the direction of the optical axis 201a. Therefore, the space between the positive and negative lens groups 22, 21 is A+α.
As described above, when spherical aberration is corrected by changing the space between two lens groups, there is a problem that spherical aberration is caused in the initial state due to a positional deviation α resulting from the gravitational displacement that depends on the orientation of the apparatus.